Mass spectrometer



July 27, 1954 w. C. WILEY MASS SPECTROMETER Filed Oct. 2, 1951 Y INVENToR. W/LL/A/I/l C. W/EV ww K g A TTOPA/E Y Patented July 27, 1954 MASS SPECTROMETER William C. Wiley, Detroit, Mich., assignor to Bendix Aviation Corporation, Detroit, Mich., a, corporation of Delaware Application October 2, 1951, Serial No. 249,318

18 Claims. l

This invention relates to apparatus for and methods of distinguishing between ions of different mass. More particularly, the invention relates to apparatus for and methods of distinguishing between ions of different mass by measuring the time required for the different ions in a pulse to travel through a predetermined distance. The invention is especially adapted to provide a sharper differentiation between ions of different mass than is provided by apparatus and methods now in use.

Some mass spectrometers now in -use differentiate between ions of different mass by accelerating a pulse of ions through a predetermined distance. The ions of relatively light mass travel faster than the ions of heavy mass and reach a collector assembly before the heavy ions. By measuring the times required for the different ions to travel through the predetermined distance the masses of the ions can be easily determined.

Certain factors cloud the measurements of the time-of-ight mass spectrometers now in use. For example, the ions in the pulse have thermal and other energy before they are accelerated towards the collector assembly. As a result of this energy, some of the ions of a given mass may be moving in a direction towards the collector assembly and other ions of the same mass may be moving away from the collector assembly at the instant that the external accelerating force is applied. Furthermore, individual ions of a given mass may be positioned in back of individual ions of the same mass. Because of their different positioning, individual ions of a given mass may reach the collector assembly at different times. Since many of the ions are not collected at the same time, the measurements are not as sharp and accurate as might be desired.

This invention provides a mass spectrometer in which any errors resulting from differences in internal energy and disposition of ions of a given mass are minimized. The spectrometer minimizes such differences by initially imposing an acceleration of moderate magnitude and subsequently imposing an acceleration of considerably increased magnitude upon the ions. As a result of the multiple accelerating forces successively applied to the ions, a relatively sharp diiferentiation is obtained between the ions of different mass.

An object of this invention is to provide a mass spectrometer for accelerating a pulse of ions through a predetermined distance and for separating the ions during flight on the basis of their mass.

(Cl. Z50-41.9)

Another object is to provide a mass spectrometer of the above character for producing a relatively sharp separation in space between ions of different mass.

A further object is to provide a mass spectrometer for minimizing any errors resulting from the positions initially assumed by ions of a given mass before the ions are accelerated towards the collector assembly.

Still another object is to provide a mass spectrometer of the above character for minimizing any errors resulting from the divergent directions of movement initially adopted by different ions as a result of their thermal and other energy.

A still further object is to provide a mass spectrometer of the above character imposing multiple accelerating forces on the ions to compensate for any differences in initial movement and disposition of individual ions before their acceleration.

Another object is to provide a method of produring a relatively sharp delineation between ions of different mass.

Other objects and advantages will be apparent from a detailed description of the invention and from the appended drawing and claims.

The single figure is a schematic view illustrating the mechanical and electrical features constituting one embodiment of the invention, the mechanical features being shown in perspective and some of the electrical features being shown in block form.

In the embodiment of the invention shown in the single gure, a cathode I0 made from a wedge-shaped strip of tungsten or other suitable material is provided. A control grid I2 is spaced a relatively small distance from the cathode and is provided With a vertical slot I4 whose median position is substantially horizontally aligned with the cathode I0. An accelerating grid I6 is substantially aligned with the control grid I2 at a relatively short distance from the grid and is provided with a vertical slot I8 corresponding to the slot I2. Similarly, a shield grid 20 is disposed substantially in alignment with the grids I2 and I6 at a relatively short distance from the grid I6 and is provided with a narrow slot 22 corresponding to the slots I4 and I8. A collector plate 24 is disposed at a relatively great distance from the grids I2, I6 and 20 and in substantial alignment with the grids.

A backing plate 26 is provided between the grid 20 and the collector plate 24, slightly to the rear of the slot 22 in the grid 20. The plate 26 is parallel to a control grid 28 having a slot 30 and is positioned a relatively short distance such as two millimeters from the grid 28. The plate 2S and the grid 28 are in electrically insulated relationship to each other. The ared mouth 32 of a conduit 34 abuts a slot 36 in a horizontal plate 38 which fits between the plate 25 and the grid 28. The conduit 34 extends from a receptacle 4S adapted to hold molecules of the different gases in an unknown mixture.

An accelerating grid 42 having a horizontal slot 4d corresponding to the slot Si! is provided substantially in alignment with the grid 28 at a relatively short distance such as two millimeters from the grid. The grid 42 is in electrically insulated relationship to the grid 28. A collector plate 45 is also substantially aligned with the grids 28 and 42 but is disposed at a relatively great distance such as 4I) centimeters from the grids for best results. A time indicator 48, such as an oscilloscope, is connected to the collector plate 55 to show the relative times at which ions of different mass reach the plate.

In the steady state condition the control grid I2 and the accelerating grid I6 have positive potentials applied to them from a suitable power supply 58. The collector plates 24 and 4t also have slightly positive voltages applied to them from suitable terminals of the power supply 5I). The cathode I0, the accelerating grid '28, the backing plate 25, the control grid 28 and the accelerating grid 42 are all substantially at ground in the steady state condition.

At predetermined times, the cathode IB and control grid I2 are adapted to receive negative pulses of the same magnitude and of relatively short duration from a pulse forming circuit 52. At approximately the same times or slightly later times, the backing plate 22 and the control grid 28 are adapted to receive positive pulses from other terminals of the pulse forming circuit 52. The pulse on the backing plate 26 is somewhat more positive than the pulse on the control grid 28 for reasons which will be disclosed in detail hereafter.

Equipment for producing a pair of pulses separated from each other by a relatively short time is known to persons skilled in the art and may be either purchased or built. For example, Model 902 of the Double-Pulse Generator manufactured by the Berkeley Scientific Company of Richmond, California, may be used to produce a pair of pulses having a variable spacing from each other. Or equipment for producing a pair of pulses having a variable time separation may be built in accordance with the principles outlined on pages 223 to 238, inclusive, of Volume entitled Electronic Time Measurements of the Radiation Laboratory Series prepared by the Massachusetts Institute of Technology.

Because of the positive voltage existing on the control grid I2 relative to the cathode Ii! in the steady state condition, electrons emitted by the cathode are attracted towards the control grid. lI'he only acceleration to the electrons is provided between the cathode and control grid, since the accelerating grid I6 has approximately the same positive potential as the control grid and the shield grid 2D is at ground. Therefore, any electrons which travel past the grid 20 are moving at a low speed and do not have suicient energy to ionize any of the gas molecules introduced to the region between the backing plate and the control grid 28.

Electrons are still attracted from the cathode Ill to the control grid I2 when equal negative pulses are applied to the cathode and control grid. These electrons are further considerably accelerated past the control grid because of the positive voltage which exists on the accelerating grid IE relative to the control grid during the negative pulse on the control grid. The electrons are so greatly accelated by the control grid and accelerating grid that. they travel past the shield grid 20 at a relatively great speed and strike molecules of gas in the region between the backing plate 2B and control grid 28 with suiiicient force to ionize some of the molecules. The gas molecules are introduced to this region from the receptacle 40, which holds the different gases in an unknown mixture.

The electrons formed by the impact of gas molecules with electrons are retained within the negative field created by the stream of electrons flowing from the cathode IIl to the collector plate 24. This stream of electrons is relatively thin in the direction perpendicular to the backing plate 26 because of the collimating action which is provided by the slots I4, I8 and 22Y and which may be provided by a magnetic field (not shown) operative on the electrons between the grid 20 and the collector plateA 24. The negative eld, or potential well, produced by the flow of electrons not only retains the ions within a relatively narrow space but also causes the number of ions so retained to be considerably increased because of the opposite charge which itv provides relative to that of the ions. By increasing the number of ions retained in a predetermined space, stronger and thus more accurate measurements can be obtained over that produced by mass spectrometers now in use. The creation and utilization of a negative field by the iiow of electrons is disclosed in detail in co-pending application Serial No. 221,554, led April 18, 1951, by me. and Ian H. McLaren.

A suflicient number of ions to saturate the negative iield` created by the flow of electrons is produced almost instantaneously. These ions are the ionsl of heavy mass and thus are collected atA the plate 46 before the ions of heavy mass. In this way, the masses of the diierent ions in the pulse can be determined by measuring the time esquired for the ions to reach the collector plate In spite of the fact that the negative stream of electrons flowing through the slots I4, I8 and 22 is somewhat restricted in width, it still has a finite thickness. This causes ions of a given mass to be positioned closer to the control grid 28' than other ions of the same mass. In addition, some of the ions are travelling towards the grid and other ions are travellingA away from the grid as a result of the random motion imparted to the ions by their thermal energy. Because of the differences in positioning and random motion between individual ions of the same mass, individual ions of a given mass reach the collector plate 46 at a different time than other ions of the same mass. These ions obscure and render somewhat inaccurate the measurementswhich are obtained.

I have found that, in a system using only" one grid to accelerate the ions, the inaccuracies resulting from the differences in positioning and random motion adopted by individual ions can be minimized if the collector plate is separated from the grid by a predetermined distance. I have further found this distance to be approximately twice the distance between the grid and the electron stream which produces the ions. Since the distance between the grid and the electron stream is small, the distance between the grid and co1- lector plate is also small. As a practical matter, this distance is so small that accurate separations in space and time between ions of different mass cannot be obtained.

By utilizing the two grids 28 and 42 instead of the one grid usually employed in time-of-flight mass spectrometers and by employing a predetermined voltage relationship between the grids and the collector plate, a mass spectrometer having improved response characteristics is obtained. Thus, voltage pulses are simultaneously applied to the backing plate 26 and the grid 28, such that the voltage on the plate 26 is moderately positive relative to the voltage on the grid. For example, a pulse of 200 volts may be applied to the plate 26 and a pulse of 150v volts to the grid. Because of the moderately positive voltage on the plate, the ions are repelled from the plate with a moderate acceleration.

The voltage pulse between the plate 26 and the grid 28 is applied for a suicient time to insure that all of the ions will be exposed to the pulse until they have reached the grid 28. Because of this, ions located at the rear of the bunch as the ions are repelled towards the grid 28 have the repelling force applied for a slightly greater length of time than ions of the same mass which are located at the beginning of the bunch. In this way, the more distantly positioned ions have a slightly greater velocity imparted to them than the ions located closer to the grid 218 to compensate for their difference in positioning. This diierence in velocity is only moderate to conform with the pulse of moderate amplitude applied between the backing plate 26 and control grid 28.

After passing the grid 28, the ions are considerably accelerated because of the relatively great diiierence between the voltages on the grids 23 and 42 when a pulse is applied to the grid 28. In the example above, a difference of 150 volts is applied between the grids 28 and 42. The accelerating force applied to the different ions of a particular mass in the region between the grids 28 and 42 is substantially constant regardless of the initial dilferences of positioning and random motion of these ions in the region between the backing plate 26 and the grid 28. This causes a substantially constant increment of energy of relatively large magnitude to be superimposed for the ions of each mass on the moderate amount of energy given to the ions between the plate 26 and the grid 28. Because of this superimposition, the ions travel through a relatively large distance before the differences in energy imposed on the ions in the region between the backing plate 26 and the grid 28 act to minimize any differences in displacement of the individual ions of each mass. The collector 46 is disposed to receive the ions at the position of optimum focussing of the ions. It should be noted that the term focussing as used herein refers to the action of bringing ions of each mass into tightly spaced groups in-the direction of ion travel.v In this way, all of the ions of each mass require a relatively short time to impinge on the collector 46.

The force imposed on the ions in the region between the backing plate 2'8 and the grid 42 acts not only to extend the position of optimum focus of the ions but also to provide a compensation for any diiferences in the random motion of individual ions. Since the ions of each mass obtain a relatively large and constant increment in energy as they pass through the region between the grids 28 and 42, this increment in energy tends to dwarf any differences in velocity resulting from the random motion of individual ions. This causes any errors resulting from the random motion of individual ions to be minimized.

Although additional grids are employed in the mass spectrometer disclosed above relative to the mass spectrometers now in use, the time required for the ions to reach the collector plate from their starting position is at least as great as inthe spectrometers now in use. The reason for this is that the ultimate velocity of the ions is no greater than in spectrometers now in use, even though they are accelerated by an increased number of grids. As a result, the ions of different mass are able to become resolved suflciently by the time they reach the collector plate, such that they can be accurately distinguished by the measurements which are made. As a matter of fact. the ions are better resolved in the spectrometer disclosed above than in the spectrometers now in use because of the focussing provided by the grids 28 and 42 to compensate for the difference in positioning and random motion adopted by individual ions. ri'he increased resolution in space in turn produces an increased resolution in the measurements which are obtained.

The utilization of two grids also cifers another advantage in that the accelerating grid 42 can be grounded. Grounding the grid 42 in effect insulates the ions from any action by the backing plate 26 and the control grid 28 after the ions have passed into the region between the grid 42 and the plate 46. Furthermore, the ground on the grid 42 minimizes any eiTect which the grid itself has on the ions after the ions have passed by the grid.

It should be realized that the ions themselves can be produced in diiferent ways than that disclosed above. Furthermore, voltage pulses of negative polarity can be supplied to the grid 28 and the grid 42 to attract the ions towards the grids rather than repelling them from the plate. It is also possible to apply a pulse of voltage on the grid 42 as well as on the plate 26 and grid 23, provided that the proper voltage relationships between the various members are maintained during the voltage pulses.

Although this invention has been disclosed and illustrated with reference to particular applications, the principles involved are susceptible.

of numerous other applications which will be The in@ apparent to persons skilled in the art. vention is, therefore, to be limited only as indicated by the scope of the appended claims.

What is claimed is:

1. A mass spectrometer, including, a backing plate, a first grid disposed relatively close to the plate, means for collecting a plurality of ions between the backing plate and the rst grid, means for applying voltage pulses to the backing plate and to the first grid of moderate relative ampli` tudes for an acceleration of the ions past the first grid to provide a focussing action onth'e ions, a second grid disposed relatively'close to the first grid' and in alignment with, the grid, means for applying. a voltage of .considerably increased amplitude relative to. the voltage pulse on the first grid to the second grid for an acceleration of: the ions through the region between the irst and' second grids to provide. an extension of the position of optimum focussing of the ions, means disposed. at a relatively great distance from the second grid and. at substantially the position of optimum focus of the ions to collect the ions, and means for indicating the relative times at which the ions of different mass are collected.

2. A mass spectrometer, including, a backing plate, a first gr-idv disposed relatively close to the backing plate, means for producing a plurality of:l ions between the backing. plate and the rst grid, means for applying voltage pulses of moderate relative amplitudes to the backing plate and tei-the rst grid for an acceleration of the ions past the first grid to. provide a focusing action onv the ions, a second grid. disposed relatively close to the first grid and in alignment with the grid, means for applying a voltage to the second grid of considerable magnitude relative to the voltage pulses on the first grid for a considerable acceleration of the ions through the region between the first and second grids to provide an extension of the position of optimum focussing ofthe ions and to minimize any errors resulting from differences in the random motion of individual ions, means for collecting the ions after they have travelled a distance past the grid corresponding substantially to the postion of optimum focus of the ions, and means for determining the time at which ions of different mass reach the collecting means.

3. A mass spectrometer, including, a backing plate,` a first grid disposed relatively close to the backing plate, a second grid disposed relatively close to the first grid, means for collecting a plurali-ty of ions in the region between the backing plate and the first grid, means for applying a voltage pulse on the backing plate, means for simultaneously applying on the first grid a voltage pulse having a duration greater than the time required for the ions to travel past the second grid, the first grid having a voltage relative to the backing plate during the pulses for a moderate acceleration of the ions past the rst grid, means for applying a voltage of considerable magnitude on the second grid relative to the voltage pulse on the first grid for a considerable acceleration of the ions through the region between the first and second grids, means disposed a relatively great distance from the second grid and at substantially the position of optimum focus of the ions to collect the ions, and means for determining the relative times at which the ions of different mass are collected.

4. A mass spectrometer, including, a backing plate, a first grid disposed substantially in alignment with the backing plate, means for collecting a plurality of ions in the region between the backing plate and the first grid, means for providing between the backing plate and the first grid a voltage pulse of a moderate magnitude and a polarity to produce a movement of the ions towards the rst grid, a` second grid disposed substantially in alignment with the first grid, means for providing between the first andsecond grids a voltage pulse of a considerably increased magnitude and a polarity to produce an accelerated movement of the ions towards and past the second grid, means disposed substantially in alignment with a second grid at a position of op- 8 timum focussing of the ions to collect the ions flowing past the grid, and means for indicating the time required for ionsA of different mass to reach the collecting means.

5. A mass spectrometer, including, a backing plate, a first, grid disposed substantially in alignment with the backing plate, means for collecting a plurality of ions in the region between the backing plate and the first grid, means for pro.- vidng between the backing plate and the rst grid a voltage pulse of a polarity and magnitude to produce a moderate acceleration of thev ions towards the iiist grid and of sufficient duration to apply an accelerating force on substantially all of the ions until their movement past theT first grid, a second grid disposed substantially in alignment with the first grid, means for providing between the first and second grids a voltage pulse of a polarity and magnitude4 to produce.v a considerably increased acceleration of the ions towards the second grid after their movement past the first grid and of sufficient durationto apply an accelerating force onv substantially all of the ions until their movement past the second( grid', meansl disposed substantially in alignment with the grids at the position of optimum focussing of the ions to collect the ions, and means for indicating the relative times at which the ions of different mass arecollected.

6. A mass spectrometer, including, a backing plate, afirst grid disposed substantially in alignment with the backing plate at a relatively small distance from the plate, means for introducinga pulse of ions into the region between the backing plate and the iirst grid, a second grid disposed substantially in alignment with the rst grid at a relatively small distance from the grid, means for applying pulses of voltage on the backing plate at periodical times, means for applying pulses of voltagel on the first grid at the same times as the application of pulse voltages on the backing plate and for at least a period of time corresponding to the travel of the ions past the second grid, the first grid having a voltage of the proper polarity and of moderate magnitude relative to the backing plate during the pulse to produce a flow of ions towards and past the first grid and to provide a compensation for differences in the positioning of individual ions, means for providing on the second grid a voltage of the proper polarity and of relatively great magnitude with respect to the rst grid to` accelerate the flow of ions towards and past the second grid and to provide an extension of the position of optimum focussing of the ions and a minimization of any errors resulting from differences in the random motion of individual ions, means disposed substantially in alignment with the grids at a relatively great distance from the grids and at substantially the position of optimum focus of the ions to collect the ions flowing past the second grid, and means for indicating the time at which ions of dierent mass reach the collecting means.

7. A mass spectrometer, including, a backingv plate, a first grid disposed substantially in align-- ment with the backing plate at a relatively short distancev from the plate, a second grid disposed substantially in alignment with the. first grid at a relatively short distance from the grid, a co1- lector disposed substantially in alignmentv with the second grid at a relatively great distance from the grid, means for forming between the. backing plate and the first. grid a pulse of ions having a, relatively restricted Width, means for imposing an electric field of moderate strenth between the backing plate and the first grid for a moderate acceleration of the ions to the rst grid, the electric ield being imposed for at least the period of travel of the ions to the first grid to provide a focussing action on the ions, means for imposing an electric field of relatively great strength between the rst and second grids for a considerable acceleration of the ions to the second grid, the electric field of great strength being imposed for at least the period of travel of the ions to the second grid to provide an extension of the position of optimum focus of the ions, the collector being disposed at substantially the position of optimum focus of the ions, and means for determining the relative times at which the ions of different mass in the pulse reach the collector plate.

8. A mass spectrometer, including, means for forming an ion pulse having a relatively restricted width, means for imposing an electrical field on the ions of sufficient magnitude and duration for a moderate increase of velocity in the ions through a first region to provide a focussing action on the ions, means for imposing an electrical field on the ions of suicient magnitude and duration for a considerable increase of energy in the ions through a second region having a width of the same order of magnitude as the rst region to provide an extension of the position of optimum focussing of the ions, means for collecting the ions at a distance past the second region corresponding to the position of optimum focussing of the ions, and means for determining the relative times at which the ions of diii'erent mass reach the collecting means.

9. A mass spectrometer, including, means for forming an ion pulse having a relatively narrow width, means for providing a moderate increase in velocity in the ions through a first region to provide a compensation for differences in the positioning of individual ions in the ion pulse, means for providing a considerable increase in energy in the ions through a second region to provide an extension of the position of optimum focussing of the ions and to provide a compensation for diiferences in the random motion of individual ions, means for collecting the ions at a distance past the second region corresponding to substantially the position of optimum focusing of the ions, and means for indicating the relative times at which the ions of different mass reach the collecting means.

10. A mass spectrometer, including, means for obtaining a plurality of ions, an ion detector, means positioned between the ion means and the detector to create a iirst region for an acceleration of the ions, means positioned between the iirst region and the detector to create a second region for an acceleration of the ions, means for moderately accelerating the ions through the iirst region until their movement past the iirst region to provide a focussing action on the ions, means for providing an increased acceleration of the ions through the second region relative to the acceleration through the first region until their movement past the second region to provide for an extension of the position of optimum focussing of the ions, the detector being disposed substantially at the position of optimum focussing of the ions, and means for indicating the relative times at which ions of diiferent mass actuate the detector,

11. A mass spectrometer, including, means for obtaining a plurality of ions, an ion detector,

means 'for Vapplying an electric iield in a rst region for an acceleration of the ions in the direction of the detector to provide a focussing action on the ions of each mass during their movement towards the detector, means for applying in a second region an electric field having an increased magnitude relative to that of the iirst region for an increased acceleration of the ions in the direction of the detector to produce an extension of the position of optimum focussing of the ions, the ion detector being located at substantially the position of optimum focussing of the ions, and means for indicating the relative times at which the ions of different mass act upon the detector.

12. A mass spectrometer, including, means for obtaining a plurality of ions, an ion detector, means for creating a first region between the ion means and the detector for an acceleration of the ions to provide focussing action on the ions, means for creating a second region between the 'first region and the detector for an acceleration of the ions to provide an extension of the position of optimum focussing of the ions, means for moderately accelerating the ions through the first region and for increasing the ion acceleration through the second region to produce a movement of the ions in the direction of the detector, the ion detector being disposed at a relatively great distance from the second region and at the position of optimum focussing of the ions and means operative upon the actuation of the detector to provide a delineation between ions of diiierent mass.

13. A mass spectrometer, including, means for obtaining a plurality of ions, an ion detector, means for producing a kinetic energy of the ions for movement of the ions through a iirst region towards the detector to provide a compensation for differences in the positioning of individual ions, means for producing an increased kinetic energy of the ions for movement of the ions through a second region towards the detector to provide a compensation for differences in the random motion of individual ions, the detector being disposed at substantially the position of optimum time-focussing of the ions and means operative upon the actuation of the detector to provide a delineation between ions of different mass.

14. A mass spectrometer, including, means for providing a plurality of ions in a relatively confined region, means for imposing an electric field on the ions in a first region to provide the ions in the back of the relatively confined region with an increased velocity relative to the ions of the same mass in the front of the region, means for imposing an electric field on the ions in a second region, the electric field in the second region being of a magnitude and extending through a distance to provide the ions with substantially constant increases in energies and with energies greater than those imparted to the ions in the first region, a detector disposed past the second region at substantially the position of optimum focussing of the ions, and means for indicating the relative times at which the ions of different mass are detected.

15. A mass spectrometer, including, a backing plate, a` first grid disposed in front of the backing plate, a second grid disposed in front of the first grid, means for providing a plurality of ions in a relatively confined region between the backing plate and the first grid, means for applying an electric field between the backing assai-.n35

@l1 plate and the first grid to limpart a greater velocity to the ions .positioned at the end Iof vthe conned region nearest to the backing :plate than to the ions of same mass at the 'end of the conned region nearest the iirst grid, means -for applying an electric eld between ythe rst and second grids relative to `the rst electric field to provide the ions with substantially constant increases in energies and with -considerably greater energies than those imparted to the ions by the first electric eld, -a-n ion rdetector `positioned in front of the second grid at substantially the position -of optimum -focus of the ions, and an indicator for determining the relative times at which the ions of Adiierent Y mass are detected.

16. A mass spectrometer, including, lmeans for providing a plurality 4of ions a relatively conned region, means :for imposing Aa` force Aon the ions in -a yiirst -region `to provide the ions in the Vback of the relatively confined region with 1an increased -velocity relative to the :ions -of the same mass in the frontof the region, means -for imposing a force on the ions in a second region of a magnitude and Vthrough a distance to provide vthe ions with substantially constant -increases in energies and with energies greater than those imparted Yto `the .ions in the rst lregion, .a detector ndisposed :past the second :region at substantially vthe :position of `optimum `focusing of the ions, and an -indicator for ldetermining the relative rtimes at which :the yions :of :different mass are detected.

17. A mass spectrometer, including, .means for providing a vplurality of ions, means -for imposing an electric eld on the ionsin -a irst region until the movement of ithe ions `past the region to provide the ions of each mass with differences 1in velocity ydependent upon the initial positioning of the ions of that mass, means for imposing an electric field on the ions in a second region until Athe movement', vof the 'ions past the region to provide the ions with substantially constant increases in energy and with energies Ygreater than those :imparted to the ions in the iirst region, a detector disposed .at the position of optimum focussing of ythe ions, and means for indicating the relative Vtimes at which the ions vof dii'erent Vmass are detected.

'18. A mass spectrometer, including, means `for providing a plurality of ions, means for imposing a 'substantially constant velectric `field Yon the ions in a rst region until -t'he movement 4of the ions past the `region, :means for imposing a rsubstantially `constant electric eld /of lincreased magnitude on the Vions in ya second Vregion until the movement of the ions .past the region, an ion -detector disposed at substantially the position 'of Noptimum ytime focussing Vof the ions, rand lmeans for indicating the relative times at which ions of diierent mass are detected.

References Cited in the file of 'this patent UNITED STATES 'PATENTS Number 

